Proofs for the Existence of God, a Comparison of Ontological and Cosmological Proof

People have debated the existence of God since the beginning of time. Many people of all faiths believe in the existence of a supreme being, but some people do not believe in God. So, how can these people be persuaded that God does exist? Anselm and Aquinas are two philosophers who attempted to prove the existence of God, though from different points of view. St. Anselm uses the ontological argument to prove God’s existence, whereas Aquinas uses the cosmological argument. This essay compares and contrasts the two arguments, stating and demonstrating how each proof works as an argument and how they differ in their approaches. It then concludes with reasons for their respective success or not as proofs for God’s existence.

According to Anselm there is a being “than which no greater can be conceived” (Melchert, 2014, p. 270). In his argument, he tries to prove God’s existence through reasoning. The ontological argument begins by defining the concept of God. For instance, if one imagines that there is God, then who is God? The very description of God simply proves that there is God. It then follows that since God can be conceived, he has to exist. It is logically absurd to imagine that God exists and not exist in reality, since it would then mean that there is a being that can be imagined and can be supreme than God. Aquinas’ cosmological argument on the other hand, states that “something that does not exist can only begin to exist through something that already exists” (Melchert, 2014, p. 281). In other words, the existence of something in the world is as a result of pre-existence of another thing, and the chain of causation traces back to one source, which is God since He is self-existent. In other words, the universe would not just exist, there must have been something that caused it to exist- and that force is God.

From the arguments, the two philosophers are basically saying the same thing only that their ways of explanation differ. Aquinas argument that Gods existence is uncaused pauses the question, why is He self-existent? The answer to this question would be, because he is all perfect, he is omniscient, omnipotent and he is eternal. This then takes us back to Anselm’s definition of God. Therefore, it is evident that both philosophers believe in the existence of a greater being who is supreme and controls the universe. However, one major difference between the two arguments is that while Anselm’s argument is solely based on reasoning with no evidence to support it, Aquinas’ argument is based on a cause-effect relationship where everything has a cause of its existence, that is, there had to be a cause for something to come into being. Another difference is that Anselm’s argument is more about what God is like, he describes God as a perfect being who is omniscient, omnipotent and eternal, but Aquinas’ argument is about how God relates to humans, how every creation traces back to Him.

All in all, the two arguments have their share of critics and supporters. The success of both arguments lies in the fact that their description of God as omnipresent, eternal and omnipotent, and His work as the source of every creation matches that that many religious convictions give of God. Even then, the arguments do not conclusively substantiate the existence of God since they can be debated. It is one’s belief and understanding of God that provides the basis for the trust and acceptance of the existence of God. Nevertheless, these arguments can help show that religious convictions are rational and interminably controversial.


Melchert, N. (2014). The Great Conversation: A Historical Introduction to Philosophy. Oxford: Oxford University Press.

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